The
system architecture and design of a continuous loop bin reproducer intended
for theme parks and large multimedia productions will be presented. Novel
features include a servoed, closed-loop tape drive, a horizontal tape storage
bin, and built-in test instruments and fault monitors. A technique using reproducer
and tape parameters to optimize the entire audio chain - from master recording
through to the theater show - will be described.

INTRODUCTION -

Murphy's
Multitrack Law - The customer will always demand more audio channels

with
higher performance and fewer failures in a smaller box for less money.

As
multimedia presentations have grown in sophistication from a single-screen
slide show to exotic productions with numerous animated figures performing
within a mechanized environment, Murphy's Multitrack Law has been at work
creating a seemingly insatiable appetite for audio reproducers with more and
more channels. These needs are commonly satisfied by three tape formats: the
NAB cartridge, reel-to-reel reproducers, and continuous loop bin reproducers.

Although
the NAB cartridge format is well suited to continuous applications, the narrow
tape width of only .25 inches imposes serious signal-to-noise and crosstalk
limitations in applications requiring more than two audio channels and one
control channel. A second set of limitations is due to the continuous layer-to-
layer friction inherent in the tape pack that frequently produces premature
mechanical failure of the cartridge. Lubricants added to the surfaces of the
tape reduce this friction, but any variation in loop tension or decrease in
lubricant efficiency can lead to speed variations, flutter, or a jammed cartridge.
In addition, loose debris from the lubricant can produce signal dropouts at
the reproduce head and traction reductions or slippage at the capstan.

Complex
shows usually require more synchronized audio tracks that can be obtained
from a single NAB cartridge. Rather than trying to synchronize several cartridges
together by means of control tracks, the common solution is to adopt a non-cartridge
format which permits more tracks to be used. Unfortunately, the format is
usually chosen to minimize the tape width, in spite of the resultant need
to squeeze many tracks onto a limited magnetic area.

If
the "squeezing" is accomplished by narrowing the track width, several
detrimental effects occur. The attendant loss of signal-to-noise ratio makes
each track noisier, which is compounded by the fact that there are also more
of these noisy tracks filling the listening area with noise. The dense spacing
of narrow tracks also exaggerates crosstalk between tracks, degrading isolation.
Small dropouts produced by tape wear constitute a larger fraction of the track
width, creating greater signal losses. Similarly, tape guiding errors produce
increased static and dynamic losses.

A
far better way - from the performance standpoint - to increase the number
of tracks is to widen the tape. Although the hardware must become more robust,
the degree of precision of the tape transport need not increase. Since much
of the cost increment for additional tracks is due to the increased cost of
the magnetic head and reproduce electronics, the overall cost/performance
ratio for a wider tape format can be significantly better than the track-narrowing
approach.

Once
a tape format has been selected, the choice between reel-to- reel and loop
bin reproducers is partially dictated by the program. If a continuous show
is required, a loop bin is preferred. Attempts to use reel-to-reel machines
in pseudo-continuous mode require either a bi-directional deck with only half
the total number of tracks in each direction or two interlaced machines that
alternately play and rewind. Both of these methods are expensive and complex.
The loop bin, on the other hand, is a very simple tape drive that need not
be as expensive as its reel-to-reel counterpart. Through proper design, the
following reel-to-reel components are rendered unnecessary:

1.Supply and takeup
motors,

2.Tape tension servos

3.Fast forward/rewind
logic

4.Motion sensors,
and

5.Tape lifters.

The
tape drive of a bin can be a single motor 'unit which offers both economy
and reliability due to its simplicity.

The
following discussion illustrates how the foregoing design considerations can
be satisfied by a bin reproducer which equals or surpasses the performance
of the best analog multitrack studio recorders available today.

OBJECTIVES

The
overall objective of the design program was to create a family of integrated
machines that were designed specifically for the multimedia market. Specific
goals were:

The
above criteria were to be executed with conservative design rules to maximize
reliability.

Two
early decisions defined the major features of the machine: the tape drive
and the bin configuration. Experimental work on a two inch bin by Altair Electronics
in 1976 had demonstrated that a closed loop differential capstan drive, patented
by 3M as the "Isoloop", was superbly suited to a wide-tape bin drive.
The outstanding benefits of the Isoloop are complete flutter and tension isolation
by the dual pinch rollers and self- generating tape tension which is solely
a function of the geometry of the multi-ridged capstan. A direct-drive Isoloop
capstan assembly was therefore the first design assumption.

Although
the earlier experimental work had used a medium capacity vertical bin, tape
wear in a vertical bin with 2400 ft. of 2 inch tape was projected to be too
severe to achieve more than 10,000 passes with less than 1 dB of signal loss
at 15 kHz. The new machine was therefore configured with a horizontal bin
intended to hold a nominal load of 2400 ft. of 2 inch tape. The smooth migration
of the six pound load of tape was to be assisted by a controlled flow of air.
The major design obstacle for the bin was expected to be the method of keeping
the tape loops upright since the stiffness of the tape is an inverse function
of the tape width.

The
audio system was configured with a relatively high impedance head feeding
directly into an integrated circuit amplifier. A very low output impedance
was chosen for the line driver to permit driving very long cables, with optional
unity-gain isolation transformers provided for noisy applications.

IMPLEMENTATION

The
first member of the desired family of bin loop products to be attempted was
the top-of-the-line unit described below which uses 2 inch tape. This approach
necessitated solving the tape drive and signal electronics problems for the
most severe set of constraints anticipated for the entire family. The design
of the other members of the family would then be a relatively straightforward
task of "down-sizing" the original system for less demanding applications.

TAPE DRIVE

A
primary objective of the tape drive was to achieve extremely good flutter
performance across the full spectrum of mechanically- and scrape-induced flutter
components. The Isoloop configuration, which has provided a benchmark in flutter
performance for audio recorders for 20 years, was modified in several respects
to maximize performance. First, the drive motor and polyester drive belt were
replaced by a high-torque, printed-rotor motor and optical tachometer mounted
directly to the capstan shaft. In addition to providing a hefty 48 ounces
of tape tension capability, the resonance-free drive assembly permits a closed
loop servo bandwidth of approximately 100 Hz.

The
reproduce head was moved to the outside of the tape loop to permit a 20% shortening
of the length of unsupported tape that produces scrape flutter. (The absence
of erase and record heads further reduces scrape flutter.) The resulting threading
path with the oxide on the outside of the loop eliminates contact between
the oxide surface of the tape and the metal surfaces of the capstan and turn-around
or reversing idler. The only components on the entire machine that touch the
tape oxide are one rotating guide, the reproduce head, two rubber pinch rollers,
and the tape cleaner. The resulting oxide wear is so low that 5,000 play cycles
of a 1.2 mil wavelength recording (12.5 kHz at 15 in/s) produced no measurable
loss in signal level.

Since
flutter testing on a reproduce-only machine requires a prerecorded test tape,
several types of 24 track recorders were tested for flutter performance and
the best unit was selected to make the test tape. Flutter readings for the
bin of less than .05% RMS for components in the frequency band from .5 Hz
to 250 Hz, and less than .07% RMS from .5 Hz to 5 kHz are nearly identical
to the readings achieved on the recorder that made the test tape. These values,
which represent a "best case" result for a first generation analog
tape, can actually be achieved in the final product if digital recorders are
employed throughout the preparation of the program material.

BIN

The
bin consists of a horizontal storage cavity that has side walls that can be
adjusted to accommodate a range of loop lengths. A hinged, transparent lid
provides convenient access to the storage cavity and tape for cleaning or
threading. A proprietary air control system uses airflow through the bin to
establish three well-defined zones within the bin. The first zone encountered
by the tape as it enters the bin is the entrance transition zone, a relatively
narrow zone stretching from side rail to side rail and extending a few inches
into the bin. Within this zone the incoming tape forms long loops reaching
toward the side rails. These loops then press against the migrating tape pack
that is being held at the downstream boundary of the entrance transition zone.

The
middle section of the bin consists of the migration zone. In this region the
loops of tape are forced toward the exit by a combination of forces due to
the airflow and the looping of the tape. Each loop is similar to a hairpin
spring, pressing against the adjacent strands. As strands are pulled from
the bin at the exit, an imbalance of force develops, forcing the tape to move
into the vacated region. New strands at the entrance end of the pack constantly
replenish the loops.

At
the exit end of the bin another transition region is established to halt the
migration of the pack before the rear wall is reached. The outermost strand
is smoothly peeled from the pack, with only a light force of one or two ounces
required to overcome the air pressure differential within the bin. The maximum
loop length that has been tested in the bin is 3400 ft., a playing time of
45 minutes at 15 in/s. The adjustable side walls were not at their maximum
position for this test, indicating the ultimate maximum load may exceed one
hour in length.

The
horizontal bin format provides a very important benefit with regard to tape
creases and wrinkles. If the bin were oriented vertically, the bottom strand
of tape would support the weight of the entire pack. During normal running,
the pack is constantly migrating, minimizing the tendency for the ends of
the loop to collapse. When the tape is stopped overnight, however, the pack
settles in the bin, causing the ends of the loops to flatten out and crease.
The creases become permanent deformations in the tape which lead to audio
dropouts.

The
horizontal bin preserves a uniform stacking between layers, preserving the
radii of the loops at the outer ends of the strands. When the bin is shut
down and the air system disabled, the pack expands slightly to fill the entrance
and exit transition zones, eliminating any tendency to crease the tape. Storage
tests up to three weeks duration at temperatures down to 50 degrees Fahrenheit
show no restart problems. Minor 'waves' in the tape are noticeable when the
tape is under very low tension, but the normal running tension in the head
area easily overcomes these distortions to

provide
uniform tape-to-head contact.

AUDIO SYSTEM

The
packaging of the audio system is a radical departure from common practice:
The entire audio chain - from the fixed azimuth reproduce head through the
amplifiers and transformer to the output connectors - can be removed as a
single unit. Both manufacturing and maintenance are thereby simplified since
the entire audio chain can be completely pre-calibrated on a test fixture
before being installed in a machine.

The
audio signal path consists of a multitrack reproduce head, a booster amplifier,
a combination equalize/current booster stage, and an output transformer. The
gain and equalization adjustments cover reference flux levels from below 100
nW/m to above 600 nW/m and standard IEC and NAB equalization for 7.5, 15 and
30 in/s. Headroom is 20 dB above 0 VU with less than .01% distortion in the
electronics. When using a flux loop equalized for IEC 15 in/s, the lower and
upper -3 dB frequencies are 15 Hz and 40 kHz. The signal-to-noise summary
in Table I indicates that the audio system is solely limited by tape noise.

TABLE
1

Signal-to-Noise Ratio

Reference Flux Level and Measurement Bandpass

200 nW/m 1040 nw/m
1040 nW/m

30
Hz-18 kHz 30 Hz-18 kHz 400
Hz-18 kHz-

Standby Mode 60
dB 74 dB
82 dB

Bulk Erased Tape 55 dB
69 dB 74
dB

Biased Tape (typical)* 52 dB
66 dB 70 dB

The
excess noise due to bias is determined by the characteristics of the tape
and master recorder, not the bin reproducer.

The
output characteristics of the audio chain have been selected to accommodate
several possible types of bin installations. If the output transmission line
from the bin to the load is short, the output transformer can be strapped
for a nominal 600 ohms. As the transmission lines become long, however, the
load on the output circuit approaches the characteristic impedance of the
line. Since most audio cables have a characteristic impedance of 90 to 200
ohms, a nominal impedance level of 150 ohms is more appropriate for long lines.
The long lines also exhibit severe variations in impedance at low and high
frequencies. To minimize errors due to these effects, the output impedance
at the output connector has been held to less than 10 ohms when strapped for
a nominal 150 ohm load. The 0 VU level for 150 ohm operation is +4 dbm150
ohms (.612 V RMS) . Applications that do not require the isolation of
the transformer permit bypassing the transformer completely, providing a capacitively-coupled
voltage-source drive.

To
avoid catastrophic shutdown of the entire audio system if one defective component
should short a power supply line, each audio channel has isolating fuses on
both power supply lines. Since the loss of a single track might go unnoticed
for a long time, a fuse monitor is included on each card to sound an alarm
that alerts an attendant. The monitor also activates a LED on the card to
aid the attendant in finding the faulty board.

An
output muting circuit has been included to minimize turn-on and turn-off transients
and to facilitate troubleshooting. A dummy load is automatically placed across
the amplifier output during muting to simulate normal operating conditions
for testing purposes.

Each
audio card also contains an alternate action pushbutton switch that connects
the output to the summing bus of the on-board monitor amplifier described
in the next section.

DIAGNOSTIC UNIT

A
common problem with modern multitrack recorders is that no provisions exist
for the operator to directly access the output of the machine. One cannot
plug in a pair of headphones to check an individual track or to scan through
all of the outputs to locate a problem. This capability, however, is essential
in a typical multimedia application where the loudspeakers are far removed
from the room containing the playback equipment. To fill this need, Altair
Electronics developed a versatile diagnostic unit that permits the operator
or maintenance technician to quickly access and measure key parameters of
the bin system.

The
function and sensitivity range are selected by a matrix of pushbutton switches
located adjacent to the multiscale meter.

The
voltmeter covers the total dynamic range of the reproducer in ten steps of
10 dB each, ranging from -70 VU to +20 VU. Since the ranges from -20 VU to
+20 VU are intended for setting levels, checking frequency response, and measuring
distortion, the frequency response has been held flat in the:audio band, with
-3 dB points at .5 Hz and 100 kHz. The ranges below -20 VU, which are intended
for noise measurements, include a bandpass filter with -3 dB points at 30
Hz and 18 kHz.

The
flutter meter and AM detector are similar to the Altair Electronics Tape and
Transport Diagnostic System (T2DS) described at this Convention
in 1980 (preprint #1637). A phase lock loop demodulator operating at 12.5
kHz provides flutter components from an in-phase phase detector and AM components
from a quadrature phase detector. Bandpass filters limit the measurement bandwidth
to .5 Hz to 250 Hz for mechanical flutter components or .5 Hz to 5 kHz for
broadband measurements which include scrape flutter. The flutter readout ranges
are .1% and .3% full scale with a residual noise below .01%; AM ranges are
1.0% and 3.0% with a residual noise below .1%. The meter is average responding,
RMS calibrated to yield flutter readings that relate to the NAB standard for
flutter.

The
outputs of the voltmeter and demodulator circuits are fed to an adjustable-gain
headphone amplifier so that the operator can use the real-time analysis capability
of the human ear to provide a qualitative evaluation of system performance.
A BNC connector in parallel with the metering circuit provides a convenient
access point for attaching outboard instruments such as distortion or spectrum
analyzers and an oscilloscope.

The
ammeter function provides a direct readout of the current in the capstan drive
motor over a range of 0 - 10 amps. Preventive maintenance procedures using
the ammeter function permit not only dynamic testing of servo motor performance
and motor protection circuits, but also testing and adjusting the amount of
motor torque required to drive each of the capstan pinch rollers. While monitoring
motor current to determine motor torque, the linkage of the pinch roller solenoids
can he adjusted to provide a uniform value of pinching force, guarantying
consistent tape tensioning at the reproduce head. The pressure and volume
of air flowing through the bin can also be adjusted by using the motor current
as an indicator of drag as the tape is pulled from the bin.

The
power supply voltage monitors consist of LED indicator's driven by voltage
comparators that verify that the audio power buses are above 95% of nominal
voltage.

CONTROL SYSTEM

The
basic operating modes consist of Play, Load, Unload, and Stop. A reflective
foil detector permits the operator to program an automatic stop at the end
of each program. A selector for internal or external speed reference for the
capstan servo permits the operator to quickly assume control of the machine
during startup or servicing.

Status
monitors check critical functions that might lead to operator errors or malfunctions.
If the operator should -fail to thread the tape correctly or to close and
secure the bin lids, a fault indicator is illuminated and bin operation is
inhibited. Other system failures that also activate the fault mode include
a loss of air supply pressure and a blown fuse in any of -the power supplies.
Indicated faults requiring operator attention, but not important enough to
shut down the system include a blown fuse on an audio card or the need to
change a roll of tape wiper fabric on the tape cleaner. Buffered outputs -from
these fault monitors are available on a status monitor connector.

APPLICATIONS INFORMATION

The
end result that is achieved with a multimedia audio system is very dependent
upon the proper choice of operating levels for each component in the audio
chain, from the original recording sessions through to the loudspeakers in
the theater. To properly utilize a reproducer with studio-grade performance,
the following analysis should be made:

First,
the signal-to-noise ratio, transfer characteristics, frequency response, and
flutter of each of the recording devices should be determined. This information
is used to construct a dynamic range envelope for each device.

Second,
the transfer levels of each step are set to optimize the dynamic range of
the final product.

Third,
the composite flutter performance can be estimated by RMS addition of the
flutter contributions of each recorder. Breaking the flutter spectrum into
bands will help to distinguish the various sources of flutter and their effects.

The
optimization of these parameters through proper choice of equipment, tape
type, and operating levels can achieve the maxi- mum possible quality in the
final program. In the extreme case, if digital audio recorders are used throughout
the mastering sequence, the final result delivered to the audience will approach
the quality of a first generation analog master tape that is normally only
heard in a recording studio. The added realism in dialog, music, and sound
effects greatly enhances the intimacy of the production, bringing more life
to the world of fantasy.

ACKNOWLEDGEMENT

The
author gratefully acknowledges the design contributions of Vern Seravic, Jay
Richardson, Roger Parker, and Filip Popovic; the critical reviews of Jack
Williams; and the tolerance and assistance of my wife, Beth, who endured the
25 hour workdays required by a very brisk schedule.